The Big City

Thousands of years in the future, Earth is entirely covered by enormous skyscrapers that are tens of miles tall and extend miles beneath the surface. There's not a bare spot of ground that isn't covered with blacktop or occupied by one of these "megascrapers." Hundreds of billions of people spend their lives entirely within these monsters of technology, living, working, shopping, relaxing. Rare is the individual who ventures onto the roofs, or even crosses over to the next building.
The concept of "outside, " of green hills and vast natural countryside, has been all but forgotten, because it's all been paved over or built upon. These millions of megascrapers comprise one vast, endless city. The Big City. Jed Morble, a Big City man, is weary of the endless rush of civilization. Weaned on fabulous tales of the Great Outdoors told to him by his long-dead grandmother, he longs for the outside, but knows he'll never find it.
So he trudges despondently through his pointless existence, until one day, he happens upon a document that points the way to a door that leads to another world-of green hills and vast natural countrysides. The only problem is, the door is deep in the heart of a bombed-out section of the city that now serves as a prison for the dregs of society. Jed decides to make the attempt. His journey will lead him through criminal territory and into a vast, unspoiled new world, where he'll clash with aliens who once decimated the Earth, and will ultimately learn a shocking secret about the new world he plans to call home.